Yeah, it kinda depends on what you want to spend. A couple of ideas:
1, a 4 channel amp, 2 channels on upgraded front speakers, and the other 2 bridged mono to a sub. Screw the rears, who the hell rides back there anyway that needs to hear the good sounds? I find myself not really even playing music when there's more than just me in the car anyway, and the location kinda sucks anyway for rear fill, being so low and right behind the front seat. Leave those stock and off of deck power.
I used this configuration for about 6 months and when I changed it I questioned the extra money I spent for the other amp.
2, a 5 channel amp, like NH said.
3, 2 amps, one 4 channel and one 2 channel. This is my current config, and it works out fine.
Everyone has a different opinion about brand. 10 different people will give you 11 different answers. I personally use PPI amps, and absolutley love them. Off the stock deck, you'll need amps that take the speaker level input. There's actually 2 different ways that companies do this: one way is a separate set of inputs for speaker level than from RCA's. PPI does it a little different, and it seens to work really well. You actually plug the speaker level directly into the RCA input. It can handle the voltage, and you don't get the noise normally associated with converters.
Ramble, ramble, ramble. Just my 2 pennies.
